Pulp screens



A. B. HUNTER PULP SCREENS Jan. 7, 1969 Filed April 1, 1966 United States Patent 1 Claim int. CI. B07b 1/18 ABSTRACT OF THE DESCLOSURE A pulp screen embodying an outer inversely frustoconical jacket and an inner vertically cylindrical screen spaced therefrom and surrounded thereby, to provide a spiral jacket of downwardly diminishing cross-section due to the provision within the space between jacket and screen of a spiral floor, the velocity of pulp-flow between the upper end source and the lower end sink remaining substantially constant, and, in combination with the foregoing (a) vanes arranged to rotate adjacent the inner screen surface for momentarily and periodically interrupting the flow of a fraction of the pulp suspension which is in the immediate vicinity of the inner and outer screensides, to prevent blinding and stapling of the screen on the outer flow-side, and (b) a 'frusto-conical base plate within said screen at the base thereof, providing an oversize accepts channel surrounding the inside perimeter of said screen and communicating with an oversize accepts discharge aperture.

The present invention relates to improvements in the screening of solid particles in fluid, and particularly liquid suspension, and predominantly, through not necessar ly, fibres in aqueous suspension, such as paper pulp and paper making stock, the present disclosure being an improvement on the subject of my United States patent application Ser. No. 427,868 filed Jan. 25, 1965'.

Having regard to the objects, purposes, and advantages of the basic inventive concept as set forth in the above cited patent applications, together with the discussion of the existing art which such applications contain, the same will not be repeated in his application except to the following abbreviated extent, and having regard to the working model of the invention which has, since the filing of the two said applications been constructed and tested according to the modifications set forth in this disclosure.

In the paper making process, and in the preparation of pulp for paper making, it is frequently necessary, at various stages, to screen the pulp fibres carried in aqueous suspension. This screening removes oversized fibres, removes foreign material, and also defiocculates the stock, by which is meant separating agglomerations of fibre which have clumped together in the liquid suspension.

Screening is accomplished by causing the suspension to flow through a screen element which may be a metal plate having perforations or slots formed therein. Alternatively it may be a woven mesh of metal wire or other material. Particles sufficiently small in size, pass through the openings in the screen element. Oversize particles are retained or held back by it, and must be removed or rejected, either to waste or to further processing.

It will readily be appreciated that some driving force is necessary to cause the liquid suspension to pass through the screen element. It will also be appreciated that means must be provided for removal of the oversize particles. It should be further understood that, due to the nature of paper making fibres, means must also be provided to prevent blocking of the openings in the screen element, since oversize fibres, or agglomerations of fibres, or fibres which have become stapled (by which is meant hooked in the screen largely due to their length) tend to block the holes, resulting in reduction of capacity of the screening device, and eventually seriously restricting, or completely blocking the flow. It should also be understood that these fibres are somewhat elastic, as well as subject to plastic deformation, and that the force applied to promote flow through the screen element, or the means provided to prevent blocking of the openings, may cause oversize particles (herein sometimes referred to as oversize accepts) to pass through the screen element by elastic or plastic deformation.

As will readily be appreciated by those skilled in the art, the value or desirability of a screening device may be judged by three major criteria. These are: its efiectiveness in removing the greatest possible proportion of oversize particles from the accepted screened stock; its effectiveness in reducing to a minimum the proportion of fibres of acceptably small size removed with the oversize fibres in the rejected stock which does not pass through the screen; and its capacity, or flow rate, for a given size of machine, or per unit area of screen element. To these, a fourth criterion may be added. This is that the device should consume the minimum amount of mechanical power per unit volume of pulp suspension screened, consistent with achieving acceptable levels of the major criteria. The consumption of minimum mechanical power implies both relatively low horsepower to drive the screening device, for a given throughput, and a relatively low pressure-drop, or loss of fluid energy through it.

The prototype or model upon which this disclosure is based was designed for 9500 US. gallons per minute flow rate. It has operated at that level, with every indication that it could be operated at considerably higher flow rates. A gallonage of 10,000 or perhaps more appears to be within its capacity. As operated at 9,500 gallons per minute, the prototype exhibited a reject rate of approximately 2 /2%, in comparison with the 10% to 15% reject rate which is considered excellent for present conventional pressure screens, and it is the inventors belief that the stated 2 /2% reject rate could be further reduced with a maintenance of good results.

The aforesaid model has been driven with approximately 60 HP. which is well below that used by conventional designs of pressure screens of comparable capacity, and is with the rotor-blade or vanes (to be described hereinafter) set at a greater angle relative to the adjacent screen-surface than is probably necessary. The pressure drop through device is only about 4 /2 p.s.i. which is also better than other commercial screens with which the inventor is familiar.

Of great importance is the excellence of product resulting from the use of the prototype herein discussed. A sample handsheet indicates that the said machine is capable of discriminating between good fibre, and oversize fibre, dirt, foreign material and the like with gratifyingly high accuracy and uniformity.

The improvements disclosed herein over the basic concept as exemplified in the inventors earlier patent applications cited above were adopted to produce a more efiicient flow passageway (between screen and the jacket, as well as tangential entry and exit), to effect more accurate control of flow rate, and in conformity with certain theoretical studies and experimental work together with the inventors practical screening experience.

With the foregoing in view, and such other objects, purposes or advantages as may become apparent from consideration of this disclosure and specification, the present invention consists of the inventive concept embodied in the method, process, construction, arrangement of parts, or new use of any of the foregoing, as herein particularly exemplified in one or more specific embodiments of such concept, reference being had to the accompanying figures in which:

FIGURE 1 is a cross-sectional elevation of the invented pulp screen on the line 11 of FIGURE 2.

FIGURE 2 is a section on the line 2-2 of FIGURE 1.

FIGURE 3 is a reduced side elevation of the invented pulp screen viewed in the direction of 33 of FIGURE 2.

FIGURE 4 is an enlarged fragmentary sectional plan on the line 44 of FIGURE 1.

FIGURE 5 is a fragmentary sectional representation substantially on the line 55 of FIGURE 1.

FIGURE 6 is a fragmentary representation substantially on the line 6-6 of FIGURE 5.

In the drawings, like characters of reference designate similar parts in the several figures.

The device of the present invention is supported in a rigid frame 10 of appropriate design, suitable for mounting on a foundation 11. The fluid suspension of solid particles, such as paper pulp, is screened by its passage radially inward through the cylindrical, perforated wall of screen 12, surrounding screen chamber 13, as it flows from a source to a sink as hereinafter described. The said perforated screen 12 is supported at its upper end by the outwardly projecting flange 14 which is welded to it at the upper end, and is positioned and relatively tightly sealed, by the guide ring 15 at the lower end.

The perforations or apertures 16 which are provided over the whole surface of screen 12, except for a narrow unperforated margin at top and bottom, may desirably be of progressively diminishing diameter toward the lower end of screen 12 as depicted in FIGURE 1. These apertures may also, if desired, be divergently tapered in the direction of screening flow as also indicated in FIGURE 6. In other words, their diameter on the inner cylinder surface may be greater than their diameter on the outer surface thereof.

Surrounding screen 12, and spaced therefrom, is a volute casing 17, surmounting, and fastened to, inverted, frusto-conical jacket 18. Within the downward tapering, annular plenum existing between screen 12 and jacket 18, a continuous spiral flow-controlling floor 19 is provided. This floor is secured to the jacket, and extends inwardly toward, and in close proximity to screen 12. Thus it will be recognized that the volute casing 17 with its upper and lower plates 20 and 21 respectively, together with jacket 18, spiral floor 19, base plate 22, and the screen 12, together define a continuous spiral passageway 23, which, in this embodiment of the present invention, makes 2% complete revolutions around the screen 12, being of progressively diminishing cross-sectional area as it progresses downwardly and around the screen 12.

The stock or suspension to be screened is conducted to the upper end of spiral passageway 23 by means of inlet channel (in general, the source) 24, so as to enter tangentially, as best observed by reference to the accompanying FIGURE 2.

The lower end of spiral passageway 23 (in general the sink) is closed by terminal plate 25, and oversize-particles reject canal 26 as best observed by reference to the accompanying FIGURES 5 and 6.

It should be noted that the dimensions, relationships, and geometrical configurations of the various parts are so chosen that, in this embodiment of the present invention, the following relationships are held as nearly as possible:

(1) The cross-sectional area of the circular entrance portion of the inlet canal 24, which is equal in area to area of inlet canal 24, as the anticipated reject flow rate bears to the desired inlet flow rate.

(3) The cross-sectional area of spiral passageway 23 on any radial plane is reduced from that at section 28, in proportion to the area of perforated plate of screen 12, which has been passed by stock at that point.

From this it will be seen that, assuming equal flow rates of accepted stock through equal areas of screen element, the linear velocity of unscreened stock through spiral passageway 23 will be constant. It will also be seen that the rejected stock leaves through reject canal 26 tangentially, and that its linear velocity will be maintained.

This has important consequences contributing greatly to the efficiency of the screen. It is well known in hydraulic theory and practise, that when a fluid moves along a curved path, the local tangential velocity at a ponit nearer the centre of curvature is greater than that at a point further from it, located on the same radial plane. In an ideal (theoretically frictionless) fluid, the product of the velocity, and the radius, equals a constant on any radial plane. The inventor has stated that the average linear velocity of the unscreened stock through passageway 23 is constant throughout its length (meaning from any radial plane to any other). However, since the velocity nearer the centre of curvature is greater than that further away, the velocity of the unscreened stock close to the screen will be greater than that which is radially more distant from it. Thus, as any individual particle approaches the screen, it accelerates in the tangential direction. Since typical pulp particles are relatively long and thin, as they are accelerated, or move closer to the screen, they will tend to line up with the direction of flow which is an important new improvement in the art of screening pulp fibres.

The spiral passageway 23, of continuously decreasing cross-section, through which the screening flow is inward, provides eflicient control of such flow, and a physical mechanism for controlling individual fibre direction as these approach the screen. Further, the maintenance of linear velocity through the smoothly diminishing crosssection of spiral passageway 23, permits high flow rates through a relatively small device, and with only a relatively low pressure drop since energy-wasting turbulence is kept to a minimum both within chamber 13, and in said passageway. Turbulence in the chamber is kept to such minimum due to the presence only of the pair of vanes to be referred to hereinafter, and their particular described, action on the suspension. In passageway 23 turbulence is kept to a minimum by the provision of the spiral floor 19 which separates each round of travel of the suspension. As a result, there is no fluid sheer between each round 'of travelling suspension which would otherwise tend to dissipate velocity energy enough through turbulence and result in marked reduction of tangential velocity towards the sink of passageway 23.

In the present embodiment, a tangential dilution inlet 28' is provided upon the length of spiral passage 23. This will not be required on all installations, and will not appear on all embodiments. With some types of pulp, where a distinct tendency for the as yet unscreened stock to increase or thicken in consistency is anticipated, the addition of dilution water or white water will promote eflicient screening. In other installations a plurality of such dilution inlets, variously spaced along the spiral passageway will prove beneficial.

The accepted suspension passing through screen 12 will still maintain some circumferential velocity. Accordingly, from FIGURE 2 it will be seen that the vanes, or foils (to be referred to) rotate in the same direction as the unscreened stock.

Therefore the rotational energy given to the suspension in chamber 13 will increase the circumferential velocity which such suspension retains. The accepted suspension moves upwardly and out, still tangentially,

through accepts canal 29 via the plenum chamber 30 closed by cover plate 30'. Hence the fluid energy in the spiralling suspension within chamber 13 will be at least partially recovered in virtue of the tangential direction of said accepts canal.

As a result it will again be observed that as far as possible all available fluid energy has been conserved, and the feature just noted also contributes usefully to the hydraulic efficiency of this embodiment, and in particular to the low pressure drop.

Since paper pulp tends to block the holes of a screen, it is deemed advisable again to describe, as applied to this embodiment, the clearing mechanism already described in the aforementioned patent applications. A shaft 31, mounted in suitable bearings 32 and driven from an external motor by means such as the sheave 33, and belting 34, enters the screen 13, through a suitable water-tight mechanical seal 35. To the shaft is keyed a hub 36, bearing upper and lower spaced support arms 37 and 37' the hub being held in place by nut 38. On the ends of the support arms are mounted two vertical vanes or foils 39, which extend the full length of screen 12.

These vanes or foils one of which is best observed in the accompanying FIGURE 4 has an outcurved trailing edge 40 which is closer to screen 12 than is the rounded leading edge 41. The foil 39 moves in relation to the screen element in the direction indicated by arrow 41. Each vane or foil 39 has a concave outer surface 42 facing the screen 12, and preferably, an opposite or inner convex surface 43. It will also be noted that the trailing edge 4% is closer to screen 12 than is the rounded leading edge 41.

The present embodiment of the invention also includes provision for adjustment of the vanes or foils: Thus foil 39 is welded to mounting blocks 44, which are attached to arms 37 by cap screws 45, and spaced by shims 46. By employing shims 46 of different thicknesses, the radial clearance 47 between trailing edge 40 and screen 12 may be increased or decreased. Similarly, by employing suitable tapered shims, the angle of attach, from leading edge 41 to trailing edge 40, may readily be varied within reasonable limits.

Finally, the present embodiment incorporates an oversize-accept outlet canal 49. This will not be included on all embodiments, but only where the operating requirements are such that the finest quality of accepts must be maintained, at some sacrifice in net throughput, or accepts flow rate, and at the expense of some extra horsepower. The oversize-accept outlet canal is connected to an oversize-accepts collecting, annular trough 50 disposed around the perimeter of the base of screen 12 and the conical, oversize-accepts directing plate 51 which also constitutes the floor of chamber 13. Such plate or process projects into the chamber 13, and functions to minimize commingling of oversize accepts with the main body of accepted stock above by directing the oversize accepts into trough 50. Plate 51 also serves usefully to enclose the shaft bearings 32.

Also, when particularly fine quality is desired, a small flow of accepts may be withdrawn from the lower portion of screen 13, through collecting trough 50, and oversize accepts outlet canal 43. These may then be re-cycled for further processing, or returned through the agency of a small external pump, to the entrance of inlet channel 24 for re-screening. As the so far unscreened stock in passage 23 moves lower down the spiral passageway, the concentration of rejectable stock becomes greater, and thus the likelihood that some oversize particles may be accepted. However, since this will be more likely to occur near the lower end of the screen these can be drawn off through the said oversize accepts outlet 49.

Notwithstanding the believed apparency of the invention as described up to this point, important novel considerations are considered to be embodied in the following amplification:

Stock consisting of solid fibres in water suspension (in the case of paper pulp or paper making stock) enters under pressure supplied by an external pump through input conduit 24- to flow in a spiral downwardly through passageway 23. During this process some of the water suspension including fibres of sufliciently small size, passes through the perforated screen wall 12 into chamber 13. Most of the suspension then passes upwardly between thte upper support arms 37 out through the accepts canal 29.

The oversize fibers are retained with the remainder of the stock in passageway 23. Such remaining stock flows spirally downward therethrough, and, as it travels, progressively more of the water and acceptably small fibers pass through the perforated screen wall 12. The remaining, unacceptable stock, progressively diminishing in volume retains however a linear velocity nearly approximating that with which it entered through input conduit 24. This is because the cross-sectional area of the spiral passageway 23 progressively diminishes downwardly, and because of the progressively shorter radial distance between the lower portion of wall or screen 12, and the lower portion of jacket 18.

This maintained linear velocity across the external surface of the screen wall 12 helps to prevent blocking or blinding of the apertures therein. However, this is not the only, nor indeed the principal provision for preventing blinding: the principal provision is the action of the vanes or foils 39. The maintained linear velocity is nevertheless an important effect in itself, and a unique feature of the present invention constituting an important improvement in the art of pressure-screening as hitherto employed. Such maintained linear velocity is however a characteristic-in-common with United States patent application Ser. No. 427,868, and Canadian patent application 921,495 already referred to herein.

It should also been noted in this context, that the suspension or stock moving spirally around and downward through annular passageway 23 at appreciable linear velocity, exhibits a marked centrifugal effect. The fibres used in paper making stock have a specific gravity which does not differ greatly from that of Water. Hence the fibres themselves will be little affected by such centrifugal effect. However, the most usual particles of foreign material found in pulp or paper stock, are particles of sand, metal fragments and the like. These of course have specific gravities considerably greater than water. Hence they are effectively flung outward from the wall 12 by the centrifugal action aforesaid.

As a result, such particles of heavy foreign matter travel downwardly in a controlled-velocity spiral until they leave tangentially through rejects canal 26, as already noted in the last but one preceding paragraph. Since pressure screens as hitherto known were not very successful in removing minute particles of heavy foreign materials which tended to pass through the screen openings with the fibres, this capability of the present invention is a further important improvement in the art of screening. Furthermore, large pieces of foreign material, such as stones, nuts and bolts, pieces of glass and the like, which may accidentally become entrained in the incoming stock or suspension, will not damage the sometimes fairly fragile screen wall portion 12 since they will be carried downwardly against the inner surface of jacket 18 under the influence of substantially maintained initial velocity, and centrifugal force. Because pressure screens hitherto have either had no provision for such objects, and were subject to damageto the screen element as a result, or else employed elaborate junktraps, or separate junk rejects outlets and the like, this feature too is considered to constitute an improvement in the art of pressure screening.

Blinding or blocking of the perforated wall portion 12 is however importantly prevented by the action within chamber 13 solely of the rotating vanes or foils 39. Screening flow through the apertures 16 of screening wall 12 is maintained by the pressure differential radially inward across it. However, each time one of the moving foils passes an individual aperture 16, there is a momentary, local interruption or reversal of the aforesaid pressure differential. This is caused by the out-curved trailing edge directing a small reversed flow or wake in the radially outward direction. In this connection it should be explained that no actual reverse flow of any appreciable amount of liquid or stock back out through the apertures 16 is necessary to clear a blocking particle pressed against the outer surface of the screen wall and held there by the pressure differential. It is sufficient for the wake from the foilmerely to neutralize the pressure differential only momentarily across the individual opening concerned, for the tangential or spiral flow through annular passageway 23 to carry away the blocking oversize fibre particle. This action, particularly in combination with the spiral sweep of the controlled-velocity stream through said passageway 23 constitutes an important feature of the present inventive concept-as-a-whole, over pressure screening techniques as hitherto employed, and again is common to the inventors of United States and Canadian applications already referred to.

The inventor is aware that vanes similar in crosssectic-nal configuration to his vanes 39 are known to rotate in a cylindrical chamber such as 12. These however have been combined with following vanes which exert suction into the cylindrical chamber in which they are located. Obviously this neutralizes the effect of the first mentioned vanes, by pulling oversize fibres through the screen aper tures. In addition, due to the absence of non-turbulent, maintained high velocity around the cylindrical screen chamber of the prior art, such prior art has failed to recognize that only a momentary interruption of pressuredrop is necessary to loosen and carry lodged particles along with the unscreened stream. Excessive energy-consuming two-way pulsation has been the result of the prior art arrangements just mentioned. Furthermore, and also of importance is the fact that the prior art foils have rotated within the concerned cylindrical chamber in the opposite direction to that of the tangential stock flow around the outside of the chamber, which obviously increases the amount of power required to rotate the vanes or foils within the screened suspension.

Various modifications can be made Within the scope of the inventive concept disclosed. Accordingly, it is intended that what is described herein should be regarded as illustrative of such concept and not for the purpose of limiting protection to any particular embodiment thereof, but that only such limitations should be placed upon the scope of protection to which the inventor hereof is entitled, as justice dictates.

What is claimed to be the present invention is:

1. A device for separating a class of particles from a fluid suspension of generally elongated particles, comprising in combination (a) a screening chamber enclosed by a vertically axial cylindrical perforated wall, (b) a frusto-conical jacket spaced from and surrounding said wall to provide a suspension-conducting passageway surrounding said wall and extending .between a tangentially arranged source of pressure and a sink spaced substantially apart from said source, said passageway including a nonplanar spiral floor extending between said wall and jacket, the special relations of said wall, jacket and floor, and the size of the apertures in said wall being adapted and arranged such that said suspension may fiow from said source to said sink over a non-planar spiral path in diminishing volume as a fraction of its passes through said wall, but at substantially constant velocity, and (c) rotary means in said chamber, operating solely to momentarily and periodically interrupt the flow of a fraction of said suspension which is adjacent the external side of said wall and in co-incidence with the perforations therein, to prevent blinding of the input side by particles which have become lodged in said perforations, said interruption causing said particles to be dislodged and thereby permitted to continue flowing past or through said perforations, the only significant counter-attraction toward the input side of said screen being that caused by the decrease in pressure upon the screen-passing fraction of said suspension, and separate discharge apertures in said chamher for accepts and oversize accepts respectively, situated at opposite ends of said chamber.

References Cited UNITED STATES PATENTS 3,145,165 8/1964 Sawdison 209379 X 3,232,436 2/1966 Nilsson 2l0415 X 3,245,535 4/1966 Cowan 209-306 X 3,261,468 7/1966 Dick 209273 X HARRY B. THORNTON, Primary Examiner.

ROBERT HALPER, Assistant Examiner.

US. Cl. X.R. 

